TheSamba.com :: View topic - Stop that fire: Marine Anti-Siphon Valves

Mon Sep 21, 2015 3:24 pm

I have been promoting the use of spring loaded check valves in both the fuel feed and return lines for years. They have been a bit hard to find at times under that name, but I recently found they were more often sold as "marine anti-siphon valves".

These would be of particular worth with either a mechanical or electric pump which doesn't not have some system to prevent gravity flow from the tank to the carb and in the return line of a FI system.

These valves would add the highest level of safely when installed close to the tank nipples and if the line between the tank and valve was wrapped with some fire resistant material.

http://www.marineengineparts.com/fuel/fuel-system-acc-y/anti-siphon-valves.html

https://www.fastfittings.com/viewproduct/635

.

Mon Sep 21, 2015 7:56 pm

Mike - I believe that valve has to be higher than the tank at the top of a loop to function properly sort of like on the dishwasher vent.

Mon Sep 21, 2015 8:56 pm

I'm having a hard time imagining how they would allow fuel to flow through then you're driving, but not flow when the engine stops.

On my boats it does seem that the valve is at the top of the tank. I think they are to prevent fuel from flowing backwards and back into the tank, as in many boats the tank is lower than the motor.

Mon Sep 21, 2015 8:57 pm

It is nothing but a spring loaded check valve and can be installed at any height.

If it were a vent, then you would not be able to suck fuel from a tank with a top outlet.

Mon Sep 21, 2015 9:37 pm

Right, but what you're saying is that this thing will prevent the tank from draining and feeding a fire.

Basically these things are a one-way valve. So how does it allow fuel to flow when the engine is running, but not when you have a fire?

The purpose on a boat is to prevent the carb from siphoning back into the (lower) fuel tank, and to keep the fuel line full of fuel.

Mon Sep 21, 2015 10:02 pm

Since an anti-siphon valve is designed to open when suction is supplied from the fuel pump, it would seem that the valve would have to be matched to the suction value of the fuel pump. So a valve that works for FI would operate at higher suction value than carbs for example. I would think that matching the values would be important for proper operation.

Wouldn't all components of the system except the tank need to be below the valve to operate properly? Would the valve offer some amount of restriction and have to be adjusted to get proper fuel flow?

Tue Sep 22, 2015 3:36 am

These anti-siphon valves are required on boats because of the extreme danger presented by fuel leakage in a marine situation. They are there as their name implies to prevent fuel from being siphoned out of the tank should there be a problem with the lines or the engine. Marine engine tend to be set low in the bilge and the tank is often placed higher causing the potential for gravity feed just like on a T2 and even if the tank is set lower than the engine if the fuel line is removed from the engine it can potentially be dropped so that it hangs lower than the fuel level in the tank.

The stock Type 4 mechanical pump had an anti-siphon valve built in to it for the same reasons that boats are required to have one. In selecting a spring loaded check valve it is not important to consider the pressure the pump puts out, but only to take into consideration the height of the tank above the engine and thus the maximum gravity pressure the valve needs to hold against. On a Bay Window this would be around 2 psi.

Tue Sep 22, 2015 7:23 am

most anti-siphon valves are put in a Tee that extends off a loop higher than the tank. When the pump pressurizes the line, the valve closes off the vent. When pressure drops to zero in the line, the valve opens and lets air into the line so that the fuel drains back to the tank instead of continuing to siphon. I can find nothing on these that indicate their behavior is different. If you have a white paper on them please post a link.

Tue Sep 22, 2015 1:06 pm

SGKent wrote: most anti-siphon valves are put in a Tee that extends off a loop higher than the tank. When the pump pressurizes the line, the valve closes off the vent. When pressure drops to zero in the line, the valve opens and lets air into the line so that the fuel drains back to the tank instead of continuing to siphon. I can find nothing on these that indicate their behavior is different. If you have a white paper on them please post a link.

You are totally on the wrong track. As I said this is not a vent of any kind it is a spring loaded check valve that opens at somewhere around 2 psi. It is very suitable for use on a bottom outlet tank such as VW Transporters have. The kind of siphon vent you are talking about would not allow an engine mounted fuel pump to work and thus could not be used in most any carbureted inboard or I/O application.

Tue Sep 22, 2015 1:29 pm

These look like a good alternative to an in-line solenoid valve.
Provided the spring pressing on the ball in a check valve can hold back the head of fuel, then nothing will come out if the fuel line downstream is broken.

I have several times found that the mechanical pump on a Type 1 can suck hard enough to lock a solenoid valve closed. So that even when there is 12 volts on it it does not open. And then you have to crack the fuel feed to the pump and let some air in to break the suction. All because of a loose power feed to the solenoid valve.

So I would be certain that e.g. a 2 psi valve would easily open with my fuel pump.

Tue Sep 22, 2015 2:48 pm

the one Mike is referring to is also sometimes called a non-return valve. It is supposed to stay shut with 20" of water in a hose above it; 25 " it opens. (That is the test, not the fluid it is designed to be used with.) That means it opens at about around .8 to .9 PSI.

Wed Sep 23, 2015 5:21 am

So someone needs to figure the PSI of 15 gallons of gasoline coming out of the fuel line.

Wed Sep 23, 2015 5:42 am

vwwestyman wrote: So someone needs to figure the PSI of 15 gallons of gasoline coming out of the fuel line.

It is the height that matters, just measure the vertical distance from the top of the tank to the location of the lowest part of the fuel system. So long as this distance is not greater than ~27" the valve should work.

From what SG Kent posted:

rated closed height for gasoline = rated height for water/specific gravity for gasoline

=20"/.75

=26.7"

Wed Sep 23, 2015 9:04 am

Good find Mike. Those would probably work on FI or Carb. I think I saw one site with a lightweight double barbed stainless one.

Wed Sep 23, 2015 9:41 am

SGKent wrote: Good find Mike. Those would probably work on FI or Carb. I think I saw one site with a lightweight double barbed stainless one.

On the return side of a fuel injection system they should be very effective as once the line on the return side of the regulator burns through there is nothing at all in the stock setup to stanch back flow of fuel from the tank. They would actually operate as a normal check valve in this usage. On the supply side the fuel pump should mostly stop the flow once the engine shuts down but having an additional valve located close to the tank outlet would a additional protection.

On a carbureted engine they should work fine in the feed line though if the pump doesn't have a prime to start with, it could be a bit difficult to get the flow to start initially. With the stock mechanical pump there use would be somewhat redundant, though if installed closer to the outlet to the tank they would still add a extra level of protection.

Wed Sep 23, 2015 9:58 am

My 76 911 with f.i has a stock check valve attached to the fuel filter. Told it is necessary to keep fuel in the lines for easier starting. Thinking VW mkI watercooled F.I. had a similar set up if I remember. Not sure if it will help ya, but just a thought.

Wed Sep 23, 2015 10:16 am

I'm firmly in the ...keep ypur stuff maintained to prevent fires...camp rather than fuel line check valves, but YFMV....

:-)

Wed Sep 23, 2015 12:01 pm

Would these cause a fuel pressure drop with the 1psi cracking pressure?

Wed Sep 23, 2015 1:28 pm

rosevillain wrote: Would these cause a fuel pressure drop with the 1psi cracking pressure?

Yes you will likely see a drop in the pressure at the inlet to the pump. Since I run a marine pump on my carbureted Bay these days I am not expecting any problems. With a low suction pressure rotary pump I can't say for sure but I would guess there would be no problem assuming the pump is mounted below the level of the bottom of the tank.